hnot sure i agree with first answer.
as how can they let air out of top if its warm in top and cold in bottom?
we had this problem, and it was because it needed draining as we had lots of sludge in the system that had built up.
our radiators with this fault had to be taken off and flushed through, its a long process, as the plumber needs to attacha hose to the radiator and flush it out and clean it and get rid of all the sludge... the stuff that came out of ours was like an oil slick... unbelievable!! howeverm once this process of taken the rad off and having it flushed out for a few days, and the system was filled with a cleaner as well, our radiators are now fully warm and not just at the top like you say yours are.
hope this helps.